The subject matter disclosed herein relates generally to a system and method for reducing harmonic content present at the output of a motor controller and, more specifically, to a system and method for monitoring harmonic current content and regulating the harmonic current content in a synchronous reference frame to reduce the harmonic current content at the output of the motor controller.
Electrical rotating machines, such as electric motors or generators, have become widespread and are found in numerous applications and configurations. Electric machines include a stationary component (i.e., the stator) and a rotating component (i.e., the rotor). In electric motors, a magnetic field is established in the rotor, for example via magnets mounted to the rotor or via an electrical current applied to or induced in a coil wound on the rotor. A second, rotating magnetic field is established as a result of a stator current produced by a controlled voltage applied to the stator. Rotation of the magnetic field in the stator causes the magnetic field in the rotor to follow the stator field, thereby causing rotation of the rotor. A shaft or other drive member is mounted to the rotor and extends outside the rotor housing providing a mechanical coupling to a device, such as a gearbox, pump, or fan that is to be driven as the rotor rotates. The amplitude and frequency of the controlled voltage is varied to achieve desired operation of the motor.
As is known to those skilled in the art, motor controllers also referred to herein as motor drives are utilized to vary the amplitude and frequency of the voltage applied to a motor to achieve desired operation of the motor. A motor controller is configured to receive power at an input, where the input power may be supplied from either an alternating current (AC) source or a direct current (DC) source. If the input power is supplied from an AC source, a rectifier section converts the AC power to DC power. The DC bus, either from the output of the rectifier section or supplied directly from the DC source is provided to a DC bus within the motor controller. A current regulator and modulation techniques are used to control an inverter section which, in turn, supplies the required current and voltage to the motor from the DC bus to achieve desired operation of the motor.
As is further known to those skilled in the art, the current regulator is commonly provided in a two-phase, synchronous reference frame. For a three-phase motor, a physical current and/or voltage may be measured in a three-phase (abc) reference frame utilizing appropriate sensing devices. The sensing devices generate a signal corresponding to the measured current and/or voltage for each phase of the motor. A transformation is applied to the measured current and/or voltage to convert the measured values from a three-phase (abc) reference frame to a two-phase (dq) reference frame. The two-phase (dq) reference frame presents current along a direct axis (d-axis) and along a quadrature axis (q-axis). The d-axis current corresponds to the flux established by the stator in the motor, and the q-axis current is proportional to the amount of torque produced by the motor.
The transformation from the three-phase to the two-phase reference frame utilizes position feedback information from the motor to obtain an electrical angle of the current applied to the motor. The electrical angle of the current is, in turn, used in the transform to convert the feedback signals into a synchronous, d-q reference frame. The synchronous reference frame is one that rotates “synchronously” or in tandem with them motor. The resulting values of the current feedback signals in the synchronous, d-q reference frame appear as “DC” values in the motor controller. In other words, the feedback signals are no longer “AC” values that alternate in a sinusoidal manner according to the frequency applied to the motor, but rather are “DC” values having a constant magnitude corresponding to the magnitude of current present in each of the “d” and “q” axes.
A two-phase synchronous frame current regulator controls current independently in both the d-axis and in the q-axis to achieve a desired flux level in the motor and a desired torque output from the motor, respectively. Because the fundamental components of the current appear as DC components in both the “d” and “q” axes, the synchronous current regulator is configured to regulate a DC value using traditional control principles rather than attempting to regulate the sinusoidally varying current being applied to the motor.
The synchronous frame current regulator is, however, not without its limitations, The synchronous frame current regulator is configured to operate within a bandwidth by which the controller gains for the regulator are defined. The synchronous frame current regulator is able to compensate for disturbances and to maintain desired operation of the current output to the motor as long as the disturbances present in the current have a frequency less than the bandwidth of the synchronous current regulator. If however, the frequency of a disturbance exceeds the bandwidth of the synchronous current regulator, the regulator is no longer able to compensate for the disturbance to generate the desired output current for the motor.
In a motor controller, a modulation routine, such as pulse-width modulation (PWM), is used to generate the output current at varying amplitude and frequency. The modulation routine controls switches to selectively connect each phase to that DC bus of the motor controller. Preferably, the switching occurs at a frequency at least an order of magnitude greater than the desired frequency of operation for the motor and, may occur in a range between 1 kHz and 20 kHz. The modulation routine, however, may introduce undesirable current components at the switching frequency or at multiples, or harmonics, of the switching frequency. These harmonic components are often at frequencies above the bandwidth of the current regulator and result in some distortion of the current waveform supplied to the motor.
Thus, it would be desirable to provide an improved current regulator that reduces distortion of the current waveform at frequencies above the bandwidth of the current regulator.
Additionally, loading and commanded operation of the motor may introduce distortion in the current waveform. As the load and/or speed of operation increases, the magnitude of current output by the motor controller approaches the capacity of the inverter and as the speed of operation increases, the fundamental frequency of the current increase, approaching the bandwidth of the current regulator. The motor controller may not be able to maintain a sinusoidal output current, but rather some ripple and/or distortion may be introduced on the waveform.
The physical construction of the motor may introduce still another source of distortion in the current waveform. In some instances, the distortion may be due to the number of stator slots and rotor poles and the winding type selected such that the motor does not include sufficient winding harmonic filtering, resulting in significant induced voltage harmonic distortion, While certain motor design features, such as skewed slots or rotor lamination or magnet profiling may improve harmonic performance, these features are costly. As a design consideration, the motor may not utilize these design features but accept as a tradeoff some current ripple or other non-sinusoidal characteristics in the current waveform. In still other applications, the motor loading may cause stator and/or rotor magnetic saturation that can produce sill additional current waveform distortion. The frequency of the distortion may occur at the fundamental frequency of the motor or at multiples (i.e., harmonics) of the fundamental frequency of the motor.
Thus, it would be desirable to provide an improved current regulator that reduces distortion of the current waveform at any identified frequency.
It is also desirable to provide an improved current regulator that identifies harmonic content present on the current waveform and to compensate the current output to the motor to reduce the harmonic content present on the current waveform.